If you simply wish to display non-animated images, then Texture images and UI RawImage controls are the way to go. However, if you want more options regarding how an image should be displayed (such as tiling and animation), the UI Image control should be used instead. This control needs image files to be imported as the Sprite (2D and UI) type.
Once an image file has been dragged into the UI Image control's Sprite property, additional properties will be available, such as Image Type, and options to preserve the aspect ratio.
If you wish to prevent a UI Sprite GameObject from being distorted and stretched, go to the Inspector window and check the Preserve Aspect option in its Image (Script) component.
